1. Field of the Invention
The present invention relates to a monolithic ceramic electronic component and a production process therefor, and to a ceramic paste and a production process therefor. More particularly, the present invention relates to a monolithic ceramic electronic component comprising internal circuit element films formed between ceramic sheets, and ceramic layers which compensate for spaces defined by step-like sections which are formed by the internal circuit element films, each ceramic layer being formed so as to have a pattern negative to that of the corresponding film, and a production process for the component; and to a ceramic paste which is advantageously employed for forming the ceramic layer, and a production process for the paste.
2. Background Art
When a monolithic ceramic electronic component, such as a monolithic ceramic capacitor, is produced, a plurality of ceramic green sheets are provided and the sheets are then laminated. In accordance with the intended function of the monolithic ceramic electronic component, an internal circuit element film, such as a conductive film or a resistive film, is formed on a specific ceramic green sheet, the film being an element of a capacitor, a resistor, an inductor, a varistor, a filter, etc.
In recent years, miniaturization and weight reduction of electronic devices such as mobile communication devices have been progressing. For example, when a monolithic ceramic electronic component is employed as a circuit element in such an electronic device, the size and the weight of the electronic component must be reduced. For instance, there has been increasing demand for a monolithic ceramic capacitor of small size and large capacitance.
A typical process for producing a monolithic ceramic capacitor is as follows. Dielectric ceramic powder, an organic binder, a plasticizer and an organic solvent are mixed together, so as to prepare a ceramic slurry. The resultant ceramic slurry is shaped into a sheet having a thickness of some tens of xcexcm by means of a doctor-blade method or a similar method on a support, such as a polyester film, which is coated with a silicone resin serving as a peeling agent, to thereby form a ceramic green sheet, and the sheet is then dried.
Subsequently, onto a main surface of the ceramic green sheet, a conductive paste is applied through screen printing, so as to produce a plurality of patterns which are separated from one another. Thereafter, the resultant sheet is dried, to thereby form internal electrodes serving as internal circuit element films on the sheet. FIG. 1 is a plan view of a portion of a ceramic green sheet 2 on which internal electrodes 1 are formed so as to be distributed at a plurality of positions as described above.
Subsequently, the ceramic green sheet 2 is peeled off the support and cut into pieces of appropriate size. Thereafter, predetermined amounts of the pieces are laminated as partially shown in FIG. 2. Furthermore, predetermined amounts of ceramic green sheets not containing internal electrodes are laminated on opposite surfaces of the resultant laminate, to thereby form a green laminate 3.
The green laminate 3 is pressed in a vertical direction with respect to a horizontal plane, and then cut into laminate chips 4 as shown in FIG. 3, the chips being of appropriate size so as to be employed as individual monolithic ceramic capacitors. Subsequently, the binder is removed from each of the chips, the resultant chip is fired, and then external electrodes are formed on the chip, to thereby produce a monolithic ceramic capacitor.
In order to reduce the size of such a capacitor and increase the capacitance thereof, the ceramic green sheets 2 and the internal electrodes 1, which are laminated, must be increased in number, and the ceramic green sheets 2 must be thinned.
However, when the laminated sheets and electrodes are increased in number and the sheets are thinned, the internal electrodes 1 are accumulated. Consequently, difference in thickness becomes more apparent between a portion at which the electrodes 1 are provided and a portion at which the electrodes 1 are not provided; or a portion at which relatively large amounts of the internal electrodes 1 are provided in a vertical direction with respect to a horizontal plane and a portion at which small amounts of the electrodes 1 are provided in the vertical direction. Therefore, for example, as shown in FIG. 3, the appearance of the resultant laminate chip 4 is deformed such that a main surface of the chip assumes a convex shape.
When the laminate chip 4 is deformed as shown in FIG. 3, a relatively large strain arises during pressing at the portion at which the internal electrode 1 is not provided or the portion at which relatively small amounts of the electrodes 1 are provided in a vertical direction with respect to a horizontal plane. In addition, adhesion between the ceramic green sheets 2 is lowered, and a structural defect such as delamination or micro-cracking tends to occur, the defect being caused by internal stress of the chip during firing.
When the laminate chip 4 is deformed as shown in FIG. 3, the internal electrode 1 is undesirably deformed, which may cause a short circuit.
The aforementioned problems may lower the reliability of the resultant monolithic ceramic capacitor.
In order to solve the aforementioned problems, for example, Japanese Patent Application Laid-Open (kokai) Nos. 56-94719, 3-74820, and 9-106925 disclose a method in which a ceramic green layer 5 is formed on a region of a ceramic green sheet 2 on which internal electrodes 1 are not formed, as shown in FIG. 4, to thereby substantially compensate for spaces defined by step-like sections which are formed by the internal electrodes 1 on the ceramic green sheet 2.
In the case in which the ceramic green layer 5 which compensates for spaces defined by the step-like sections is formed, when a green laminate 3a is formed as partially shown in FIG. 5, there is no substantial difference in thickness between the portion at which the electrodes 1 are provided and the portion at which the electrodes are not provided; or between the portion at which relatively large amounts of the internal electrodes 1 are provided in a vertical direction with respect to a horizontal plane and the portion at which small amounts of the electrodes 1 are provided in the vertical direction. Therefore, as shown in FIG. 6, the resultant laminate chip 4a tends not to be undesirably deformed as shown in FIG. 3.
Consequently, the aforementioned structural defect such as delamination or micro cracking, or short circuit due to deformation of the internal electrode 1 tends not to occur, thereby enhancing the reliability of the resultant monolithic ceramic capacitor.
The aforementioned ceramic green layer 5 which compensates for spaces defined by the step-like sections has a composition similar to that of the ceramic green sheet 2, and the layer 5 is formed by applying a ceramic paste containing dielectric ceramic powder, an organic binder, a plasticizer and an organic solvent onto the green sheet 2. In order to form the layer 5 through printing at high accuracy so as to attain a thickness (e.g., 2 xcexcm or less) which is equal to that of the internal electrode 1, the dispersibility of the ceramic powder in the ceramic paste must be high.
In connection with the foregoing, for example, Japanese Patent Application Laid-Open (kokai) No. 3-74820 discloses a method for preparing a ceramic paste in which ceramic powder is dispersed by use of a three-roll mill. However, it is difficult to increase dispersibility of the ceramic powder through use of a three-roll mill only.
Japanese Patent Application Laid-Open (kokai) No. 9-106925 discloses that a ceramic slurry for forming a ceramic green sheet 2 is prepared by mixing dielectric ceramic powder, an organic binder and a first organic solvent having a low boiling point, and the resultant slurry is employed for forming a ceramic green sheet 2; and that the slurry is mixed with a second organic solvent having a boiling point higher than that of the first organic solvent, and the resultant mixture is heated so as to remove only the first organic solvent from the mixture, to thereby prepare a ceramic paste for forming a ceramic green layer 5 which compensates for spaces defined by step-like sections.
When the ceramic paste is prepared through at least two mixing steps, the dispersibility of the ceramic powder is improved to some extent. However, since the slurry or paste contains the organic binder, the viscosity of the slurry or paste becomes high during mixing. This imposes a limitation on improving dispersibility of the ceramic powder when an apparatus such as a ball mill is employed.
As already mentioned, there is a requirement for high dispersibility of the ceramic powder contained in the ceramic paste employed for forming the ceramic green layer 5 which compensates for spaces defined by step-like sections, the layer being very thin and having a thickness equal to that of the internal electrode 1. When the thickness of the internal electrode 1 decreases, the ceramic powder must exhibit a correspondingly higher dispersibility.
Even when dispersibility of the ceramic powder is low in the ceramic green layer 5, the ceramic green sheet 2 provided on the layer 5 may compensate for such low dispersibility to some extent. However, when the thickness of the sheet 2 decreases, the sheet cannot fully compensate for such low dispersibility.
Therefore, as the development of a monolithic ceramic capacitor of small size and large capacitance has progressed, there has been a growing need for high dispersibility of the ceramic powder contained in the ceramic green layer 5 which compensates for spaces defined by step-like sections.
In order to enhance dispersibility of the ceramic powder in the ceramic paste during mixing, the viscosity of the paste may be decreased. However, when the amount of the aforementioned organic solvent of low boiling point is increased in order to decrease the viscosity of the paste, removal of the solvent following dispersion of the ceramic powder requires a prolonged period of time.
When a monolithic ceramic capacitor is produced by firing a green laminate 3a including three different elements, i.e., ceramic green sheets 2, internal electrodes 1 and ceramic green layers 5 which compensates for spaces defined by step-like sections which are formed by the electrodes, these three elements shrink during debinding and then firing. Consequently, stress due to differences in the physical properties of these three elements arises in the capacitor, and the stress may cause structural defects within the capacitor.
In the process disclosed in Japanese Patent Application Laid-Open (kokai) No. 9-106925, the organic binder contained in a ceramic paste for forming a ceramic green layer 5 which compensates for spaces defined by step-like sections is the same as that contained in a ceramic slurry for forming a ceramic green sheet 2. Briefly, the organic binders are dissolved in the same organic solvent. Therefore, when the ceramic green sheet 2 is laminated on the ceramic green layer 5 which has been dried, the organic binder contained in the dried layer 5 is dissolved with the organic solvent contained in the sheet 2, and thus the layer 5 may be impregnated with the organic solvent. In view of the above, the ceramic paste disclosed in Japanese Patent Application Laid-Open (kokai) No. 9-106925 is not necessarily suitable for a ceramic paste for a very thin ceramic layer such as the ceramic green layer 5 which compensates for spaces defined by step-like sections.
Hereinabove, problems to be solved are described with reference to a monolithic ceramic capacitor, but similar problems may be involved in other monolithic ceramic electronic components, such as a monolithic inductor.
In view of the foregoing, an object of the present invention is to provide a process for producing a monolithic ceramic electronic component which enables solution of the above-described problems; and a monolithic ceramic electronic component produced through the process.
Another object of the present invention is to provide a ceramic paste which is suitable for forming a very thin ceramic green layer such as the above-described ceramic layer which compensates for spaces defined by step-like sections; and a process for producing the paste.
Accordingly, the present invention provides a process for producing a monolithic ceramic electronic component. The process comprises the following steps.
Firstly, a ceramic slurry, a conductive paste and a ceramic paste are provided.
Subsequently, a plurality of composite structures are formed, each comprising a ceramic green sheet which is formed from the ceramic slurry; internal circuit element films which are formed by applying the conductive paste partially onto a main surface of the ceramic green sheet so as to provide step-like sections; and a ceramic green layer which compensates for spaces defined by the step-like sections, the ceramic green layer being formed by applying the ceramic paste to the region on the main surface of the ceramic green sheet on which the element films are not formed, so as to substantially compensate for the spaces.
Subsequently, the composite structures are laminated, to thereby form a green laminate. Then, the green laminate is fired.
The process for producing a monolithic ceramic electronic component comprises the above essential steps, wherein the ceramic paste for forming the ceramic green layer which compensates for step-like sections comprises ceramic powder, an organic solvent, and an organic binder. A characteristic feature of the process of the present invention resides in the organic binder contained in the ceramic paste.
That is, the organic binder is a mixture formed by physically mixing two different organic binders, such as a mixture of polyvinyl butyral and a cellulose ester, a mixture of a polyacrylate and a cellulose ester, a mixture of polyvinyl butyral and polyvinyl acetate, or a mixture of polyvinyl butyral and a polyacrylate; or a copolymer predominantly comprising an alkyl acrylate and/or an alkyl methacrylate.
When the aforementioned copolymer predominantly comprising an alkyl acrylate and/or an alkyl methacrylate is employed as the organic binder, the copolymer may contain, as a copolymerization element, a reactive monomer having a carboxyl group, an alkylene oxide group (RO)n, a hydroxyl group, a glycidyl group, an amino group or an amido group.
Preferably, the ceramic paste comprising the organic binder is produced through the following steps.
The production process for the ceramic paste comprises a first dispersion step in which a first mixture containing ceramic powder and a first organic solvent undergoes processing for providing a primary dispersion; and a second dispersion step in which a second mixture containing an organic binder and the first mixture which has undergone the first dispersion step undergoes processing for providing a secondary dispersion. It should be noted that the organic binder is added during the second dispersion step. In addition to the first organic solvent, a second organic solvent having a relative evaporation rate lower than that of the first organic solvent is employed. The second organic solvent may be added during the first dispersion step or the second dispersion step. Alternatively, the second organic solvent may be added during the first dispersion step, and further added during the second dispersion step. After completion of the second dispersion step, the second mixture is heated, to thereby selectively remove the first organic solvent from the mixture.
In the present invention, the ceramic slurry for forming the ceramic green sheet preferably contains ceramic powder which has a composition substantially the same as that of the ceramic powder contained in the ceramic paste for forming the ceramic green layer which compensates for spaces defined by step-like sections.
Preferably, a dielectric ceramic powder is contained in the ceramic slurry and ceramic paste. In this case, when internal circuit element films are internal electrodes which are arranged so as to provide capacitance therebetween, a monolithic ceramic capacitor can be produced.
Preferably, a magnetic ceramic powder is contained in the ceramic slurry and ceramic paste. In this case, when internal circuit element films are formed as hook-shaped conductive films, a monolithic inductor can be produced.
The present invention also provides a monolithic ceramic electronic component produced through the above-described production process.
The present invention also provides a ceramic paste as described above, and a process for producing the paste.